Recent Advances in Pharmaceutical Cocrystals: A Focused Review of Flavonoid Cocrystals

Cocrystallization is currently an attractive technique for tailoring the physicochemical properties of active pharmaceutical ingredients (APIs). Flavonoids are a large class of natural products with a wide range of beneficial properties, including anticancer, anti-inflammatory, antiviral and antioxidant properties, which makes them extensively studied. In order to improve the properties of flavonoids, such as solubility and bioavailability, the formation of cocrystals may be a feasible strategy. This review discusses in detail the possible hydrogen bond sites in the structure of APIs and the hydrogen bonding networks in the cocrystal structures, which will be beneficial for the targeted synthesis of flavonoid cocrystals. In addition, some successful studies that favorably alter the physicochemical properties of APIs through cocrystallization with coformers are also highlighted here. In addition to improving the solubility and bioavailability of flavonoids in most cases, flavonoid cocrystals may also alter their other properties, such as anti-inflammatory activity and photoluminescence properties.

Importance of the Positively Charged σ-Hole in Crystal Engineering of Halogenated Polypeptoids

Crystalline nanosheets formed by amphiphilic block copolypeptoids with halogenated phenyl side chains were imaged at the atomic-scale using cryogenic transmission electron microscopy (cryo-TEM). In general, the polypeptoid molecules adopt V-shaped configurations in the crystalline state, and adjacent molecules can pack with one another in either parallel or antiparallel arrangements, depending on the chemical composition. The halogen bond, which can have characteristic energies ranging from 1 to 5 kcal/mol, is commensurate with the parallel configuration. However, cryo-TEM images show that chains in the halogenated crystals were in the antiparallel configuration. Molecular dynamics (MD) simulations show that positively charged σ-holes, which are characteristic of halogen atoms covalently bonded to carbon atoms, play an important role in determining crystal geometry. Parallel and antiparallel configurations exhibited similar stability in simulations when standard force fields that only account for the electronegativity of halogen atoms were used. However, including the σ-hole in the simulations resulted in a destabilization of the parallel configuration. This combination of imaging and simulation, which has played an important role in structural biology, has the potential to improve our understanding of factors that govern noncovalent interactions in synthetic materials.

Interplay of halogen and hydrogen bonding in a series of heteroleptic iron(iii) complexes

The impact of the halogen substituent on supramolecular preferences that influence packing is explored in a series of heteroleptic iron(iii) complexes.

Influence of Halogen Substituent on the Self-Assembly and Crystal Packing of Multicomponent Crystals Formed from Ethacridine and Meta-Halobenzoic Acids

In order to determine the influence of halogen substituent on the self-assembly of the 6,9-diamino-2-ethoxyacridinium cations and 3-halobenzoate anions in the crystals formed from ethacridine and halobenzoic acids, the series of ethacridinium meta-halobenzoates dihydrates: ethacridinium 3-chlorobenzoate dihydrate (1), ethacridinium 3-bromobenzoate dihydrate (2), and ethacridinium 3-iodobenzoate dihydrate (3), were synthesized and structurally characterized. Single-crystal X-ray diffraction measurements showed that the title compounds crystallized in the monoclinic P21/c space group and are isostructural. In the crystals of title compounds, the ions and water molecules interact via N–H⋯O, O–H⋯O and C–H⋯O hydrogen bonds and π–π stacking interactions to produce blocks. The relationship between the distance X⋯O between the halogen atom (X=Cl, Br, I) of meta-halobenzoate anion and the O-atom from the ethoxy group of cation from neighbouring blocks and crystal packing is observed in the crystals of the title compounds.

Refinement of labile hydrogen positions based on DFT calculations of 1H NMR chemical shifts: comparison with X-ray and neutron diffraction methods

Numerous gas phase electron diffraction, ultra-fast electron diffraction, X-ray and neutron diffraction experiments on β-dicarbonyl compounds exhibiting enol-enol tautomeric equilibrium, with emphasis on acetylacetone and dibenzoylmethane, have so far been reported with conflicting results on the structural details of the O-HO intramolecular hydrogen bond and resulted in alternative hypotheses on the intramolecular hydrogen bond potential function either a double minimum potential corresponding to two tautomeric forms in equilibrium or a single symmetrical one. We demonstrate herein, firstly, that the DFT calculated OH ¹H NMR chemical shifts of acetylacetone and dibenzoylmethane exhibit a strong linear dependence on the computed OO hydrogen bond length of ∼-50 ppm Å^-1 and as a function of the O-HO bond angle of ∼1 ppm per degree, upon the transfer of the hydrogen atom from the ground state toward the transition state. Secondly, the refinement of labile hydrogen atomic positions in intramolecular hydrogen bonds based on the root-mean-square deviation between experimentally determined and DFT calculated ¹H NMR chemical shifts in solution can provide high resolution structures of O-H and O(H)O bond lengths and O-HO bond angles with an accuracy of ∼10^-2 Å and ∼0.5°, respectively. Thirdly, the calculated ¹H NMR chemical shifts in solution of the two ground state tautomers in equilibrium of acetylacetone and dibenzoylmethane are in excellent agreement with the experimental value, even for moderate basis sets for energy minimization. In contrast, the single symmetrical structure in a strongly delocalized system is a transition state with calculated ¹H NMR chemical shifts which strongly deviate from the experimental value. Fourth, the DFT calculated ground state O-H bond lengths of acetylacetone and dibenzoylmethane are in quantitative agreement with the literature data which take into account the effect of quantum nuclear motion. The DFT structural results are critically discussed with respect to the state-of-the-art variable temperature X-ray and neutron diffraction methods.

Halogen bonding driven crystal engineering of iodophthalonitrile derivatives

Various halogen bonding driven crystal structures can be obtained by simple modifications of iodophthalonitrile derivatives.

The effects of halogen bonding and molecular design on the self-assembly of coordination polymers of Mn(iii)-tetraarylporphyrin with axial bridging ligands

Two new coordination polymers of Mn(iii)-tetra(iodophenyl)porphyrin with isonicotinate and pyrimidine-5-carboxylate linkers have been synthesized. The effect of halogen-bonding interactions on their self-assembly in the crystalline bulk and on a graphite surface has been discussed.

Synthesis of an extended halogen-bonded metal–organic structure in a one-pot mechanochemical reaction that combines covalent bonding, coordination chemistry and supramolecular synthesis

We describe a four-component one-pot mechanochemical reaction which combines the formation of covalent bonds, coordination bonds and halogen bonds to obtain an extended structure based on halogen-bonded metal–organic units.

Bis[2,4-dichloro-6-(ethyl-imino-meth-yl)phenolato-κN,O]nickel(II)

In the title compound, [Ni(C(9)H(8)Cl(2)NO)(2)], the Ni(II) ion lies on an inversion centre and is coordinated in a slightly distorted square-planar geometry by an N and an O atom from two symmetry-related bidentate 2,4-dichloro-6-(ethyl-imino-meth-yl)phenolate ligands. In the crystal structure, there are short Cl⋯Cl distances of 3.506 (1) and 3.350 (1) Å.